Optoelectronic device

ABSTRACT

An optoelectronic device and method of making the same. The device comprising: a substrate; an epitaxial crystalline cladding layer, on top of the substrate; and an optically active region, above the epitaxial crystalline cladding layer; wherein the epitaxial crystalline cladding layer has a refractive index which is less than a refractive index of the optically active region, such that the optical power of the optoelectronic device is confined to the optically active region.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 15/120,861, filed Aug. 23, 2016, which is aNational Stage Patent Application of International Patent ApplicationNo. PCT/GB2015/050520, filed Feb. 24, 2015, which (i) claims priority toU.S. Provisional Patent Application No. 62/057,818, filed Sep. 30, 2014,(ii) claims priority to United Kingdom Patent Application No.GB1420064.6, filed Nov. 11, 2014, and (iii) claims priority to UnitedKingdom Patent Application No. GB1403191.8, filed Feb. 24, 2014, and thepresent application is a continuation-in-part of International PatentApplication No. PCT/EP2017/080216, filed Nov. 23, 2017, which (i) claimspriority to U.S. Provisional Patent Application No. 62/426,117, filedNov. 23, 2016, (ii) claims priority to U.S. Provisional PatentApplication No. 62/427,132, filed Nov. 28, 2016, (iii) claims priorityto U.S. Provisional Patent Application No. 62/528,900, filed Jul. 5,2017, (iv) claims priority to United Kingdom Patent Application No.GB1711525.4, filed Jul. 18, 2017, (v) claims priority to U.S. patentapplication Ser. No. 15/700,055, filed Sep. 8, 2017, now U.S. Pat. No.10,185,203, issued Jan. 22, 2019, and (vi) claims priority to U.S.patent application Ser. No. 15/700,053, filed Sep. 8, 2017, now U.S.Pat. No. 10,133,094 issued Nov. 20, 2018; U.S. patent application Ser.No. 15/700,055 claims the benefit of U.S. Provisional Patent ApplicationNo. 62/528,900, filed Jul. 5, 2017 and claims priority to United KingdomPatent Application No. GB1711525.4, filed Jul. 18, 2017; and U.S. patentapplication Ser. No. 15/700,053 claims the benefit of U.S. ProvisionalPatent Application No. 62/528,900, filed Jul. 5, 2017 and claimspriority to United Kingdom Patent Application No. GB1711525.4, filedJul. 18, 2017. All of the applications listed in this paragraph areincorporated herein by reference.

FIELD

The present invention relates to optoelectronic devices, andparticularly to optoelectronic devices with no buried oxide layer orother insulating layer between an optically active region and asubstrate.

BACKGROUND

Conventional optoelectronic devices (for example, electro-absorptionmodulators or EAMs) comprise a waveguide on a base that is generally asemiconductor substrate such as a silicon substrate. Waveguides builtupon this base comprise three layers: a core layer, a bottom claddinglayer, and an upper cladding layer; which are configured to guide alight signal through the core layer by total internal reflection. Thecore layer is a light-transmitting medium, which is conventionally athin (relative to an active material discussed below) silicon layerlocated on top of an insulating layer such as a buried oxide or BOXlayer. The BOX layer, as part of the waveguide (bottom cladding) islocated on top of the silicon substrate and functions to confine thelight into the light-transmitting medium. An optoelectronic device maycomprise a waveguide with an optically active region (also referred toas an active waveguide), for example an electro-absorption medium,deposited in a cavity in the silicon layer (i.e. atop the BOX layer).Typically, a thin silicon layer is left on the bottom of the cavitybetween the BOX layer and the optically active region as a crystal seedfor the active material to be grown epitaxially. Both the silicon seedlayer and the BOX layer may function as the bottom cladding for theactive waveguide. Usually, the epitaxial growth for the active materialneeds a further active material seed layer located upon the silicon seedlayer in order to obtain a high quality crystal structure of the desiredactive region. For example, a seed layer of germanium may be grown whenan active layer of silicon-germanium is to be grown. The uniform andcontinuous silicon layer must be kept relatively thin so as to maintainthe coupling efficiency between a passive waveguide (e.g. anon-optically active waveguide) and the active waveguide at a usefullevel. In previous optoelectronic devices, the thickness of the siliconlayer is around 0.2 μm.

To fabricate a known EAM as described above from a silicon wafer, thesilicon layer above the BOX must be etched to a thickness of around 0.2μm from an initial thickness of around 3 μm. It is difficult to do thisconsistently, and so problems with yield may arise.

Known optoelectronic devices which operate at 1310 nm wavelengths sufferfrom a number of issues. For example, in Mach-Zehnder interferometerbased devices operating at this wavelength have a very large footprinton a photonic circuit, which can result in a very large parasiticcapacitance. Moreover, the driver circuit for such a device is verycomplicated and will often require a distributed electrode andtransmission line design. Quantum-confined Stark effect devicesoperating at this wavelength show a high polarization dependency (whichmeans that the performance difference such as insertion loss andextinction ratio for TE and TM modes can be out of the acceptable range)as well as a high sensitivity to manufacturing process tolerance.Operational bandwidth is also limited in a trade-off with extinctionratio i.e. for a given extinction ratio the maximum bandwidth iscorrespondingly limited.

Until now, it has been accepted that a silicon seed layer and a BOXlayer are necessary beneath the optically active region as bottomcladding in order to make the optoelectronic device function. However,the inventors have realised that the silicon seed layer and the BOXlayer are not necessary and can be replaced by other material that has acrystal structure with a lower refractive index than that of theoptically active region.

SUMMARY

The invention generally relates to an optoelectronic device with aregrown or epitaxial crystalline cladding layer below an opticallyactive region. For example, a silicon-on-insulator (SOI) wafer, where aportion of the buried oxide has been removed, and a cladding layerregrown in its place.

Accordingly, in a first aspect, the invention provides an optoelectronicdevice, comprising an optically active region and a substrate; wherein abottom cladding layer positioned between the optically active region andthe substrate, and directly adjacent to the substrate, is provided by anepitaxial crystalline layer. In this way, there is no buried oxide layerbetween the optically active region and the substrate. The substrate maybe a silicon substrate.

The bottom cladding layer may be an epitaxial crystalline layer, forexample a silicon or SiGe epitaxial cladding layer; and an upper surfaceof the epitaxial cladding layer may abut a bottom surface of theoptically active region.

In a second aspect, the invention provides an optoelectronic device,comprising: a substrate; an epitaxial crystalline cladding layer, on topof the substrate; and an optically active region, above the epitaxialcrystalline cladding layer; wherein the epitaxial crystalline claddinglayer has a refractive index which is less than a refractive index ofthe optically active region, such that optical power of theoptoelectronic device is confined to the optically active region.

The epitaxial crystalline cladding layer, may also be referred to as aregrown cladding layer or an epitaxial cladding layer or an epitaxialsemiconductor layer. By regrown, it may be meant that duringmanufacturing, a pre-existing cladding layer may have been removed and areplacement regrown in its place. For example, when the pre-existingcladding layer is a buried oxide layer, it may be removed and a claddinglayer may be regrown which is formed of a material which is not buriedoxide. By epitaxial, it may be meant that a crystalline structure of thecladding layer is a function of a crystalline structure of the substrateupon which the cladding layer is located. The cladding layer may also bereferred to as a crystalline cladding layer. By epitaxial crystalline,it may be meant that the cladding layer has been epitaxially grown usinga semiconductor material. By the term optical power, it may be meantthat an optical signal or a majority of the optical signal of the deviceis confined to the optically active region or that the main opticalsignal passing through the device passes through the optically activeregion.

Advantageously, the coupling loss between a passive waveguide (connectedor connectable to the optically active region) and the optically activeregion according to the invention is smaller than that between the sameregions in a prior art device that has a buried oxide layer. Generally,the cladding layer functions by having a contrasting (lower) refractiveindex than the active region and hence, the light is confined in theactive region. Moreover, it is possible to tune the height of thecladding layer to optimize mode match which can result in lower devicelosses and higher yield in device fabrication. Furthermore, the absenceof a buried oxide layer between the cladding layer and the opticallyactive region can remove or diminish RF parasitic capacitance (e.g. byremoval of C_(ox) capacitance) which may result in a higher devicespeed.

By regrown, it may be meant that the cladding layer is provided as alayer grown from the substrate or an intermediate layer e.g. it may bean epitaxial or crystalline cladding layer. The cladding layer may bedirectly on top of the substrate. Alternatively, the cladding layer maybe separated from the substrate by a seed layer (for example, agermanium seed layer), or the cladding layer may function as a seedlayer. The seed layer can help ensure that the crystal structure of thecladding layer and/or the optically active region is of good quality andhave low defect density, as well as encouraging growth along the correctcrystal axis. In some embodiments, the seed layer is kept thin, forexample substantially equal to the thickness of the BOX in the passivewaveguides, which is around 400 nm, as this will help minimise loss.Generally, the top surface of the seed layer should be substantiallyequal to the top surface of the BOX layer in the adjacent passivewaveguides. The substrate may be a silicon substrate.

The optically active region may be one of: an electro-absorptionmodulator; a photodiode; or an avalanche photodiode. Theelectro-absorption modulator may operate by the Franz-Keldysh orQuantum-confined Stark effects. Where the optically active region isformed of any one of: SiGe, SiGeSn, GeSn, InGaNAs, or InGaNAsSb theoptically active region may be an electro-absorption modulationoperating by the Franz-Keldysh effect.

The device may further comprise an insulating layer, disposed on top ofthe substrate and horizontally adjacent to the cladding layer, and thecladding layer may be formed of a material which is different from theinsulating layer. The insulating layer may be a buried oxide layer. Byhorizontally adjacent, it may be meant that the insulating layer isadjacent on a different side of the cladding layer than the substrate.Horizontal may refer to a direction perpendicular to a height of thedevice i.e. parallel to an upper surface thereof.

The cladding layer may be formed of silicon. The silicon may beepitaxially grown silicon. The cladding layer may be formed of amaterial which is not a buried oxide. The cladding layer may be formedfrom silicon germanium (SiGe), germanium (Ge), indium phosphide (InP),or may be a combination of a SiGe and Ge layer. Where there is a seedlayer between the substrate and the cladding layer, the cladding layermay further act to isolate any optical losses which might be caused bythe seed layer. The optically active region may be formed of SiGeSn,GeSn, InGaNAs, InGaNAsSb, or SiGe having a first composition and thecladding layer may be formed of the same material having a secondcomposition different from the first composition. In this example, theremay be a germanium seed layer or a SiGe layer, functional as a seedlayer, disposed between the cladding layer and the silicon layer. Wherea seed layer is provided between the cladding layer and the siliconsubstrate layer, the seed layer may sit directly on top of the siliconsubstrate layer and the cladding layer may sit directly on top of theseed layer. The optically active region may be formed of SiGeSn, and thecladding layer may be formed of SiGe.

The device may further comprise a germanium seed layer between thecladding layer and the optically active region. Alternatively, the seedlayer may be SiGe or SiGeSn.

The device may further comprise an insulating layer (for example, aburied oxide layer), disposed on a first and/or second horizontal sideof the cladding layer, wherein the cladding layer has a height from thesubstrate which is substantially equal to that of the insulating (e.g.buried oxide) layer. The cladding layer may have a height from thesubstrate (i.e. measured orthogonally to the plane of the substrate)which is 0.4 μm for a 3 μm silicon-on-insulator (SOI) platform, or whichmay be more for different SOI platforms. The cladding layer is formed ofa material different from the buried oxide layer. The insulating (e.g.buried oxide) layer may extend under a portion of a slab of theoptically active region i.e. the regrown or epitaxial crystallinecladding layer may not be as wide in a horizontal direction as theoptically active region, the optically active region also comprising arib under which the cladding layer is disposed.

The optically active region may be disposed within a cavity of asilicon-on-insulator layer which is disposed above the substrate. Theoptically active region may be formed of any of: SiGe, SiGeSn, an SiGemultiple quantum well (MQW) epitaxially grown stack, or an InP-based MQWepitaxially grown stack. The optically active region may be formed ofSi_(x)Ge_(1-x-y)Sn_(y), where 5%≤x≤20% and 1%≤y≤10%. Such a device maybe operable at an optical wavelength of 1310 nm, and may have a lengthof the optically active region of between 30 μm and 60 μm. The devicemay be driveable at a voltage of between 1.8 V-2 V which may be measuredpeak to peak and applied as a reversed bias. Advantageously, such adevice is less polarization dependent than previous 1310 nm devices andthe performance of the device is generally less sensitive to processvariation.

The optically active region may be capped with a capping layer.

The device may further comprise an input waveguide, coupled to a firstside of the optically active region; and an output waveguide, coupled toa second side of the optically active region; wherein the interfacebetween the input waveguide and the optically active region and theinterface between the output waveguide and the optically active regionare at an angle greater than 0° relative to a guiding direction of theinput waveguide and/or output waveguide.

The optically active region may include a SiGe optical waveguide, andthe waveguide may comprise a junction region and a plurality ofelectrodes for providing a bias across the junction to enable use of theFranz-Keldysh effect, Quantum-confined Stark effect, or control of thephase of light traveling through the junction region via dispersion. Theoptically active region may include a SiGeSn optical waveguide, and thewaveguide may comprise a junction region and a plurality of electrodesfor providing a bias across the junction to enable use of theFranz-Keldysh effect or control of the phase of light traveling throughthe junction region via dispersion.

In an embodiment, the optically active region may include a waveguideridge, and may have: an upper surface and a lower surface; a lower dopedregion, located at and/or adjacent to at least a portion of the lowersurface of the optically active region, and extends laterally outwardsfrom the waveguide ridge in a first direction; an upper doped region,located at and/or adjacent to at least a portion of the upper surface ofthe waveguide ridge of the optically active region, and extendslaterally outwards from the waveguide ridge in a second direction; andan intrinsic region located between the lower doped region and the upperdoped region.

In this embodiment, a first electrode may contact the lower doped regionat a first contact surface and a second electrode may contact the upperdoped region at a second contact surface; the first contact surface maybe laterally offset from the waveguide ridge in a first direction; andthe second contact surface may be laterally offset from the waveguideridge in a second direction. The first and second contact surfaces maybe aligned with one another along a lateral plane.

In this embodiment, the upper doped region may comprise a first dopedzone and a second doped zone. The dopant concentration in the seconddoped zone of the upper doped region may be higher than the dopantconcentration in the first doped zone of the upper doped region; and thesecond doped zone of the upper doped region may comprise the secondcontact surface. The first doped zone of the upper doped region may beat and/or adjacent to the upper surface of the waveguide ridge of theoptically active region (OAR), and the second doped zone is located at aposition which is laterally displaced from the waveguide ridge in thesecond direction.

In this embodiment, the lower doped region may comprise a first dopedzone and a second doped zone. The dopant concentration in the seconddoped zone of the lower doped region may be higher than the dopantconcentration in the first doped zone of the lower doped region; and thesecond doped zone of the lower doped region may comprise the firstcontact surface. The first doped zone of the lower doped region may belocated directly underneath the OAR; and the second doped zone of thelower doped region may be located within the OAR, laterally displacedfrom the waveguide ridge, the second doped zone of the lower dopedregion may have an upper surface which comprises the first contactsurface, and a lower surface which is in direct contact with the firstdoped zone of the lower doped region. The second doped zone of the lowerdoped region may be located within a portion of the OAR having a reducedheight. The portion of the OAR having a reduced height may be a portionof the OAR which has been etched before the dopant species of the lowerdoped region is added.

In this embodiment. The first doped zone of the lower doped region maybe located directly underneath the OAR. The OAR may include a slab whichextends in the first direction, the slab may exhibit a via through itsthickness at a location laterally displaced from the waveguide ridge inthe first direction; and the second doped zone of the lower doped regionmay be located within the first doped zone, directly underneath the via.

In this embodiment, the lower doped region may be partially adjacent tothe lower surface of the OAR and may be partially migrated into the OARat the lower surface.

In this embodiment, the upper doped region may be fully located withinthe OAR.

In this embodiment, the OAR may be formed from an electro-absorptionmaterial in which the Franz-Keldysh effect occurs in response to theapplication of an applied electric field.

In this embodiment, the OAR may be formed from a light absorbingmaterial which is suitable for generating a current upon detection oflight and electrons to be swept out when a voltage bias is appliedacross the upper and lower doped regions.

In this embodiment, the optically active region may include a waveguideridge, a first slab on a first side of the waveguide ridge and a secondslab on a second side of the waveguide ridge, the OAR may have an upperand surface and a lower surface. The lower doped region may be locatedadjacent to a portion of a lower surface of the OAR; the lower dopedportion may also extend laterally along and adjacent to the first slabof the OAR, away from the ridge in a first direction. The upper dopedregion may be located within at least a portion of an upper surface ofthe ridge of the OAR, and may extend laterally outwards along the secondslab of the OAR in a second direction. The lower doped region, which maybe located adjacent to a portion of a lower surface of the OAR, maymigrate into the OAR at the same portion of the lower surface of theOAR.

In an embodiment, the optically active region may include a ribwaveguide modulation region, the rib waveguide modulation region having:a ridge extending from the cladding layer; a first slab region at afirst side of the ridge and a second slab region at a second side of theridge; and wherein: a first doped region extends along: the first slabregion and along a first side wall of the ridge, the first sidewallcontacting the first slab region; and a second doped region extendsalong: the second slab region and along a second sidewall of the ridge,the second sidewall contacting the second slab region.

In a different embodiment, the optically active region may include a ribwaveguide modulation region, the rib waveguide modulation region having:a ridge extending from the cladding layer, and all of the ridge, or atleast a portion of the ridge being formed from a material which isdifferent from the material of the cladding layer; wherein the ribwaveguide modulation region includes a first slab region at a first sideof the ridge and a second slab region at a second side of the ridge. Afirst doped region may extend along: the first slab region and along afirst side wall of the ridge, the first sidewall contacting the firstslab region. A second doped region may extend along: the second slabregion and along a second sidewall of the ridge, the second side wallcontacting the second slab region.

In an embodiment, the optically active region may include a ribwaveguide modulation region, the rib waveguide modulation region having:a ridge extending from the cladding layer, at least a portion of theridge being formed from a chosen semiconductor material which isdifferent from the material of the cladding layer; a first slab regionat a first side of the ridge and a second slab region at a second sideof the ridge; and either the first slab region or the second slab regionis the material of the cladding layer; and wherein: a first doped regionextends along: the first slab region and along a first side wall of theridge, the first sidewall contacting the first slab region; and a seconddoped region extends along: the second slab region and along a secondsidewall of the ridge, the second sidewall contacting the second slabregion.

In a different embodiment, the optically active region may furtherinclude a rib waveguide modulation region, the rib waveguide modulationregion, the ridge waveguide modulation region having: a silicon base,disposed on top of the cladding layer; a ridge extending from thesilicon base, at least a portion of the ridge being formed from a chosensemiconductor material which is different from the material of thesilicon base. The silicon base may include a first slab region at afirst side of the ridge, and a second slab region at a second side ofthe ridge. A first doped region may extend along: the first slab regionand along a first sidewall of the ridge, the first sidewall contactingthe first slab region. A second doped region may extend along: thesecond slab region and along a second sidewall of the ridge, the secondsidewall contacting the second slab region.

In this, or the previous embodiment, the chosen semiconductor materialmay be silicon germanium (SiGe), silicon germanium tin (SiGeSn), GeSn,InGaNAs, InGaNAsSb, or a metal alloy of silicon, germanium or SiGe. Thechosen semiconductor material may be a GeSn alloy grown on Si or on anIII-V semiconductor.

In this, or the previous embodiment, the device may further comprise afirst electrical contact located on the first slab region of the siliconbase or cladding layer and a second electrical contact located on thesecond slab region of the silicon base or cladding layer. The firstdoped region may be n doped and the second doped region may be p doped.

In this embodiment, the ridge may comprise: a lower ridge portion incontact with and extending away from the base; the base and lower ridgeportion being formed from silicon; and an upper ridge portion in contactwith and extending away from the lower ridge portion, the upper ridgeportion being formed from the chosen semiconductor material. The firstdoped region which extends along the first sidewall may include a lowersidewall portion located at the first ridge portion and an uppersidewall region located at the second ridge portion. The second dopedregion which extends along the second sidewall may include a lowersidewall portion located at the first ridge portion and an uppersidewall region located at the second ridge portion. The lower sidewallportions and slab regions may have a higher dopant concentration thanthe upper sidewall portions. The lower sidewall portions may have ahigher dopant concentration than the upper sidewall dopantconcentrations, and the slab regions of the doped regions may have ahigher dopant concentration than the lower sidewall portion. A distance(d_(np2), d_(pp2)) by which the first and second lower sidewall portionsextend into the ridge may be greater than the distance (d_(n), d_(p)) bywhich the first and second upper sidewall portions extend into theridge.

In this, or the previous embodiment, the device may include an input ribwaveguide coupled to the input of the rib waveguide modulation region tocouple light into the rib waveguide modulation region, and an output ribwaveguide coupled to the output of the rib waveguide modulation regionto couple light out of the rib waveguide modulation region.

In this, or the previous embodiment, the height of the base or claddinglayer (h₂) and the height of the lower sidewall portions (h₃) may bechosen such that the mode center of the rib waveguide is located at thesame height above the base or cladding layer as the mode center of theinput and/or output waveguide(s).

The optically active region may include a waveguide ridge; and awaveguide slab; and at least one of: a sidewall of the ridge; a portionof the slab; the entirety of the slab; a portion of the ridge adjacentto the slab; both sidewalls of the ridge; may be formed of crystallineor amorphous silicon and contains dopants. The remainder of thewaveguide slab may be formed from SiGe, SiGeSn, GeSn, InGaNAs, InGaNAsSbor germanium.

In this embodiment, it may be that a portion of the slab and an adjacentsidewall are formed of crystalline or amorphous silicon. In such anexample, the portion of the slab and the adjacent sidewall may containdopants of a same species. In addition, it may be that a further portionof the slab and a further adjacent sidewall are formed of crystalline oramorphous silicon, and so both sidewalls and both portions of the slabeither side of the ridge may be formed of crystalline or amorphoussilicon. In this example, the further portion of the slab and thefurther adjacent sidewall contain dopants of a different species tothose contained in the first portion of the slab and the first adjacentsidewall. Alternatively, the entire waveguide slab may be formed ofcrystalline or amorphous silicon, as well as a portion of the ridgewhich is adjacent to the slab, such that the crystalline or amorphoussilicon forms an inverted CT′ shape. In such examples, a portion of thewaveguide slab may contain dopants of a first species and a differentportion of the waveguide slab may contain dopants of a second species.In addition to this, it may be that one or both sidewalls are formed ofcrystalline or amorphous silicon. In examples where only one sidewall isformed of crystalline or amorphous silicon (containing dopants), theother sidewall is formed from an active material (for example SiGe orSiGeSn) which is also doped. The sidewalls in such examples may containdopants, and respectively contain dopants of a different species.Advantageously, such arrangements are easier to manufacture. Moreover,devices having at least one electrical contact disposed on a doped Siportion display improved series resistance which can improve thebandwidth and linearity of optical power.

In a third aspect, the invention provides an optoelectronic device,formed on a silicon-on-insulator wafer comprising a substrate, aninsulating layer (e.g. a buried oxide layer), and a silicon-on-insulatorlayer, comprising: a cladding layer, formed of a different material tothe material of the insulating layer, on top of the substrate; and anoptically active region, above the cladding layer; wherein the claddinglayer has a refractive index which is less than a refractive index ofthe optically active region such that an optical mode of theoptoelectronic device is confined to the optically active region, andwherein the insulating layer does not extend below the optically activeregion. The optically active region may be formed of: SiGeSn, GeSn,InGaNAs, or InGaNAsSb. The optically active region may be formed ofSiGe. The cladding layer may be formed of: silicon or SiGe.

The optoelectronic device of the third aspect may have any of thefeatures discussed with relation to the optoelectronic device of thesecond aspect.

In a fourth aspect, the invention provides a method of manufacturing anoptoelectronic device from a silicon-on-insulator wafer comprising asubstrate, an insulating layer (e.g. a buried oxide layer), and asilicon-on-insulator layer, the method comprising the steps of: etchinga cavity into the wafer, such that a depth of the cavity extends to atleast an upper surface of the substrate; growing a cladding layer ontothe upper surface of the substrate; growing an optically active materialonto the cladding layer, wherein the cladding layer has a refractiveindex which is less than a refractive index of the optically activematerial; and etching the optically active material so as to form anoptically active region above the cladding layer. The optically activematerial may comprise SiGe. The optically active material may compriseany one of: SiGeSn, GeSn, InGaNAs, or InGaNAsSb. The cladding layer maycorrespond to the cladding layer discussed above, and may have any ofthe features disclosed in relation thereto.

Advantageously, a better uniformity in slab height can be achieved dueto there being less etch variability. Therefore the devices may beeasier to manufacture and/or result in higher yields. Moreover, as thecladding layer is grown directly on the silicon substrate (or on a seedlayer on the silicon substrate) the fabrication process can be fasterthan conventional methods. This can lead to a fabrication process whichmay result in higher density of devices on a chip, higher yield ofdevices produced, and a lower cost for integrating photonic circuitsinto a silicon-on-insulator platform.

The step of etching a cavity into the wafer may comprise: a firstetching step, where the insulating (e.g. buried oxide) layer is used asan etch-stop; and a second etching step, where the substrate is used asan etch-stop.

The method may include a step, before growing the cladding layer, ofgrowing a seed layer onto the upper surface of the substrate such thatthe cladding layer grows from the seed layer.

The method may include a further step after growing the optically activematerial of planarizing the grown optically active material.

The method may include a step, after growing the optically activematerial and before etching the optically active material, or disposinga hard mask on top of at least a part of the grown optically activeregion.

The method may include a step, after etching the optically activematerial, of: doping the optically active region with dopants of a firstand second species, so as to provide an electro-absorption modulator.

In a fifth aspect, the invention provides an optoelectronic devicecomprising: a waveguide slab, disposed on top of an insulating layer(e.g. a buried oxide layer); and a waveguide ridge, disposed on top ofthe waveguide slab; wherein at least one of: a sidewall of the ridge; aportion of the slab; the entirety of the slab; a portion of the ridgeadjacent to the slab; both sidewalls of the ridge; are formed ofcrystalline or amorphous silicon and contain dopants.

In this embodiment, it may be that a portion of the slab and an adjacentsidewall are formed of crystalline or amorphous silicon. In addition, itmay be that a further portion of the slab and a further adjacentsidewall are formed of crystalline or amorphous silicon, and so bothsidewalls and both portions of the slab either side of the ridge may beformed of crystalline or amorphous silicon. Alternatively, the entirewaveguide slab may be formed of crystalline or amorphous silicon, aswell as a portion of the ridge which is adjacent to the slab, such thatthe crystalline or amorphous silicon forms an inverted CT′ shape. Inaddition to this, it may be that one or both sidewalls are formed ofcrystalline or amorphous silicon.

A first sidewall and second sidewall of the waveguide ridge may berespectively doped with dopants of a first species and dopants of asecond species. A first side of the waveguide slab, adjacent to thefirst sidewall, and a second side of the waveguide slab, adjacent to thesecond sidewall, may be respectively doped with dopants of a firstspecies and dopants of a second species. The remainder of the waveguideridge and waveguide slab may be formed of SiGe or germanium.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 shows a plan view of an optoelectronic device;

FIG. 2A shows a cross-sectional view of the device of FIG. 1 along theline A-A′;

FIG. 2B shows a cross-sectional view of the device of FIG. 1 along theline B-B′;

FIGS. 3A-3P show various manufacturing steps;

FIG. 4 shows a variant device;

FIG. 5 shows a variant device;

FIG. 6 shows a variant device;

FIG. 7 shows a variant device; and

FIGS. 8-14 show variant structures for the optically active regionand/or device.

DETAILED DESCRIPTION

FIG. 1 is a plan view of an optoelectronic device 104 as disposed on achip 100. An input waveguide 101 is operable to guide a light signalalong direction 102 and through an interface 103 into the device. Theinterface between the input waveguide and the device is at an angle α₁relative to the guiding direction 102 of the light. The angle α₁ maytake a value between 0° and 10°. In some embodiments α₁ is approximately8°.

The light signal, having passed through the interface into the device104, enters an optically active region (OAR) 105 where it may beprocessed or modified. For example, the optically active region may beany of: a photodiode; an electro-absorption modulator; or an avalanchephotodiode. Depending on the nature of the optically active region, thelight signal may then exit the OAR and device 104 via interface 108,into an output waveguide 106.

The output waveguide 106 guides light in direction 107, and theinterface 108 may be at an angle α₂ relative to the guiding direction107 of the light in the output waveguide. As with angle α₁, the angle α₂may take a value between 0° and 10°. In some embodiments α₂ isapproximately 8°, and is generally equal to α₁.

FIG. 2A is a cross-sectional view of the device 104 shown in FIG. 1,along the line A-A′. The device 104 comprises a silicon substrate 201which is a lowermost layer of the device. Disposed on top of thesubstrate are two buried oxide (BOX) layers 202 a and 202 b; and,between the buried oxide layers, is a cladding layer 203 which may be Sior SiGe. On top of the cladding layer is an optically active region 105which is connected on either side to the input waveguide 101 and outputwaveguide 106. The interfaces 103 and 108 between the waveguides and theOAR are shown. A capping SiO₂ layer 206 is shown in this figure.Notably, the buried oxide layers 202 a and 202 b do not extend under theoptically active region 105. The buried oxide layers may extendpartially under a slab of the rib waveguide, i.e. under doped regions210 and 211.

FIG. 2B is across-sectional view of the device 104 shown in FIG. 1,along the line B-B′. This figure shows in more detail an example of theoptically active region 105. Disposed on top of the cladding layer is awaveguide that comprises an intrinsic part 205, a first doped region 208and a second doped region 209 which are formed from the same material asthe intrinsic part 205. Alternatively, either doped regions 208 or 209,or both doped regions 208 and 209, may be formed from a differentmaterial from the intrinsic part 205 such as Si or SiGe. The dopedregions 208 and 209 extend along an upper surface of the cladding layer203, and up sidewalls of the intrinsic part 205. The dopants in thefirst doped region are of a different species to the dopants in thesecond doped region.

A first portion 210 of the first doped region 208 is heavily doped incomparison to the remaining first doped region. This portion 210 isconnected to an electrode 232 a, which extends through the SiO₂ cappinglayer 206. Similarly, a second portion 211 of the second doped region209 is heavily doped in comparison to the remaining second doped region.This portion 211 is connected to a second electrode 232 b, which extendsthrough the capping layer 206. The OAR 105 is generally located in acavity of a silicon layer, the cavity being partially defined by siliconsidewalls 207 a and 207 b. The intrinsic part 205 in this example isundoped, and so the OAR can be described as a p-i-n junction. As theintrinsic part 205 extends away from the cladding layer, it may bedescribed as a proud or rib waveguide where the rib is provided by theintrinsic part 205 and a part of first 208 and second 209 doped regionswhich extend up the side of the intrinsic part 205 and the slab isprovided by a part of the doped regions 208 and 209 which extends alongthe upper surface of the cladding layer 203. The rib waveguide may havea height of around 2.8 μm as measured from the upper surface of thecladding layer, and the slabs may have a height of around 200 nm. Thewidth of the rib waveguide (i.e. the horizontal distance between theparts of the first and second doped regions which extend up the side ofthe intrinsic part 205) may be around 0.8 μm. The cladding layer may beapproximately 400 nm thick (i.e. as measured from the uppermost surfaceof the silicon substrate to the uppermost surface of the claddinglayer). In such examples, the coupling efficiency from the inputwaveguide into the waveguide 205 has been computed as approximately 99%for TE mode and 98.7% for TM mode.

The cladding layer 203 functions to confine light signals entering theOAR into the rib waveguide. It does so primarily by being formed of amaterial having a refractive index which is less than that of the OAR.For example, the cladding layer may be formed of a silicon layer whichmay be epitaxially grown or deposited using chemical vapour depositionwhich can have a refractive index of 3.3 to 3.8. In. In contrast, thewaveguide and/or OAR may be formed primarily of silicon germanium (SiGe)which can have a refractive index of 4.0-4.7. This change in refractiveindex across the interface between the OAR and cladding layer mayprovide enough index contrast (i.e. Δn) to confine the light signals tothe waveguide. It is notable that good confinement can be achievedwithout a buried oxide layer below the OAR, as discussed above.

FIG. 3A-3P discuss manufacturing steps to provide a device as shown inFIGS. 2 and 3, shown along the cross-section B-B′. In a first step,shown in FIG. 3A, a silicon-on-insulator wafer is provided. The wafercomprises a silicon substrate 201, a buried oxide layer 202 disposedthereon, and a silicon-on-insulator layer 207. Next, as illustrated inFIG. 3B, a first mask 212 is disposed over a region of thesilicon-on-insulator layer and then the unmasked region is etched downto the buried oxide layer 202. This results in a cavity 213 in thesilicon-on-insulator layer which is partially defined by sidewalls 207 aand 207 b. Next, the buried oxide layer in between the sidewalls 207 and207 b is etched away and the first mask is removed, resulting in astructure shown in FIG. 3C. The cavity is now at least partially definedby sidewalls of the silicon-on-insulator layer 207 a and 207 b as wellas sidewalls 202 a and 202 b of the remaining buried oxide layer.

Next, as illustrated in FIG. 3D, an insulating liner 215 a and 215 b maybe provided along the sidewalls 207 a and 207 b of the cavity 213.Indeed, in some embodiments there is no liner provided along thesidewalls of the cavity. The liner may extend along the top of thesidewalls 207 a and 207 b as illustrated. After the liner has beenprovided, a cladding layer 203 is grown onto the silicon substrate 201as shown in FIG. 3E. The cladding layer may be an epitaxially grownsemiconductor (for example silicon) layer, and may be referred to as anepitaxial crystalline layer. The liner may ensure that the claddinglayer grows with a generally homogenous crystal structure, as it mayonly grow from the silicon substrate and not from the sidewalls. As anoptional extra step after regrowing the cladding layer, the insulatingliner 215 a and 215 b, which may have been provided along the sidewalls207 a and 207 b of the cavity 213, may be removed.

After the cladding layer has been provided, the optically active region217 is grown as shown in FIG. 3F. Prior to this step, a seed layer maybe grown on top of the cladding layer. This can benefit the formation ofthe optically active region. The optically active region may be providedby the blanket deposition of germanium into the cavity 213. Afterdeposition, the optically active region 217 is planarized by, forexample, chemical-mechanical polishing such that an uppermost surface ofthe OAR is level with an uppermost surface of the liner 215 a and 215 b,as shown in FIG. 3G. If there is no liner, then the uppermost surface ofthe OAR would be level with the uppermost surface of the sidewalls 207 aand 207 b.

Next, as shown in FIG. 3H, a second mask 218 is provided over a portionof the OAR, and the unmasked region is etched to provide slabs 220 a and220 b of the waveguide. The unetched region provides a rib waveguide 219as discussed above. This completes the key manufacturing steps forproviding the optically active region.

As a further step, shown in FIG. 3I, a capping layer 221 is providedover the OAR. This capping layer is sufficiently thin that dopants canbe implanted into regions of the OAR through the capping layer. Forexample, as shown in FIG. 3J, a third mask or photoresist 222 isprovided a region of the device. The unmasked region is then exposed todopants 223 of a first species, so as to dope a region 208 of theoptically active region. In this example, the dopants are injected intoa region of the slab 220 a which is unmasked as well as a sidewall ofthe rib of the waveguide. The dopants may be, for example, boron and sothe region is doped with a p type species of dopant. The third mask isthen removed.

Similarly, as shown in FIG. 3K, a fourth mask or photoresist 224 isprovided over a region of the device. The unmasked region is thenexposed to dopants 225 of a second species, so as to dope a region 209of the optically active region. In this example, dopants are injectedinto a region of the slab 220 b which is unmasked as well as a sidewallof the rib waveguide. The dopants may be, for example, phosphorus and sothe region is doped with an n type species of dopant. The fourth mask isthen removed.

So as to decrease the electrical resistance of the first 208 and second209 doped regions, further doping may be performed as will be discussed.In FIG. 3L, a fifth mask or photoresist 226 is disposed over a region ofthe device, and an unmasked region is exposed to further dopants 227 ofthe first species. This results in a first heavily doped region 210within the first doped region 208. This region may be described as p++doped relative to the p doped region 208. The fifth mask is thenremoved. Similarly, as shown in FIG. 3M, a sixth mask or photoresist 228is provided over a region of the device, and an unmasked region isexposed to further dopants 229 of the second species. This results in asecond heavily doped region 211 within the second doped region 209. Thisregion may be described as n++ doped relative to the n doped region 209.The sixth mask is then removed.

As a further step shown in FIG. 30, a seventh mask 230 may be providedover a region of the device, and the unmasked regions may be etched soas to remove portions of the capping layer 221 above the first 210 andsecond 211 heavily doped regions. This produces vias 231 a and 231 b.The seventh mask is then removed. In a final step, shown in FIG. 3P,electrodes 232 a and 232 b are provided which respectively contact thefirst 210 and second 211 heavily doped regions through the vias. Anelectric potential can be applied via electrodes 232 a and 232 b,resulting an electric field which passes horizontally across thewaveguide 219. The device may therefore utilize the Franz-Keldysh effectto modulate the amplitude of light signals passing through.

A variant device is shown in FIG. 4, where a germanium seed layer 401 isdisposed between the cladding layer 416 and the silicon substrate 201.As is also shown in this figure, the seed layer 401 is positioned withina cavity of the silicon substrate 20, such that the cladding layer 416is disposed in a similar position to previous embodiments. Like featuresare indicated by like numerals. As will be appreciated, the additionalfeatures shown in FIG. 2B may also be present in this device, but forthe sake of clarity are not shown.

Similarly, a further variant device is shown in FIG. 5. Here, the firstdoped region 501 extends only part of the way up the sidewall of thewaveguide 219. As will be appreciated, the additional features shown inFIG. 2B may also be present in this device, but for the sake of clarityare not shown. This device is suitable for providing a bias across thejunction to enable control of the phase of light traveling through thejunction region via dispersion. The structure of the device and itsmethod of manufacture are similar to that disclosed in WO 2016/0139484titled “Waveguide Modulator Structures”, the entire contents of which isincorporated herein by reference.

Another variant device is shown in FIG. 6. Here, a further silicon layer601 is doped to provide a first doped region 602 and a second dopedregion 604. Alternatively, the further silicon layer 601 is not used andthe first doped region 602 is made in the cladding layer 203. Theyrespectively include a first heavily doped region 603 and a secondheavily doped region 605. In contrast to the previous devices, the firstdoped region 602 does not extend up a sidewall of the waveguide 219, butinstead extends along a lowermost surface of the waveguide 219. Further,the second doped region 604 extends along an uppermost surface of thewaveguide 219. Therefore, when a voltage is applied to electrodes 232 aand 232 b, a vertical electric field can therefore be provided acrossthe waveguide 219 in contrast to the horizontal electric field inprevious examples. The structure of the device and its method ofmanufacture are similar to that disclosed in WO 2017/081196 A1 titled“An optoelectronic component”, the enter contents of which isincorporated herein by reference.

A further variant device is shown in FIG. 7. In this Figure, a device isshown comprising a ridge modulation region with a height h_(wg); theridge modulation region being made up of a base 701 manufactured from afirst waveguide material M₁ and a ridge 702 manufactured from a secondwaveguide material M₂ which is different from the first waveguidematerial.

The base 701 includes a first slab region extending away from a firstsidewall of the waveguide ridge in a first direction, and a second slabregion extending away from a second sidewall of the waveguide ridge in asecond direction; the second direction being opposite the firstdirection.

The device includes a first doped region, the first doped regionincluding a first doped slab region 713 a and a first doped sidewallregion extending along the first sidewall of the waveguide.

As shown in the Figure, the ridge of the waveguide is formed from alower ridge portion 712 a and an upper ridge portion 712 b. The lowerridge portion is in contact with and extends away from the base; thebase and lower ridge portion both being formed from the first materialM₁. The upper ridge portion is made from the second material M₂ locatedon top of the lower ridge portion in that it is in contact with andextends away from the lower ridge portion.

The first doped sidewall region extends along the entire sidewall of theridge including both the lower ridge portion 712 a and the upper ridgeportion 712 b. The first doped sidewall region therefore comprises afirst lower sidewall portion 713 b which extends along the firstsidewall at the lower ridge portion of the ridge; and a first uppersidewall portion 713 c which extends along the sidewall at the upperridge portion of the ridge.

Similarly, at the second side of the rib waveguide, the device comprisesa second doped slab region 714 a and a second doped sidewall regionextending along the second sidewall of the waveguide. The second dopedsidewall is made up of a second lower sidewall portion 714 b whichextends along the second sidewall at the lower ridge portion of theridge; and a second upper sidewall portion 713 c which extends along thesidewall at the upper ridge portion of the ridge.

The dopant concentration at the doped slab regions and the lower dopedsidewall regions are higher than those of the upper doped sidewallregions. In the example shown, the first doped slab region and the firstlower sidewall doped region are n++ doped, whilst the first uppersidewall is n doped; the n++ doped region typically contains at leastone to two orders of magnitude more dopant per cm³ as compared to the ndoped region. The second doped slab region and second lower sidewalldoped region are p++ doped whilst the first upper sidewall is p doped.

In the example shown, the first material M₁ is formed from silicon (Si)and the second material M₂ is formed of silicon germanium (SiGe) orsilicon germanium tin (SiGeSn). However, it is envisaged that thestructure of this embodiment could equally be applied to other suitableoptical materials. Examples of suitable dopant concentrations for anM₁/M₂ structure of Si/SiGe or Si/SiGeSn are shown in Table 1 below:

TABLE 1 Doping range Doping type [1/cm³] n 1e15-1e18 p 1e15-1e18 n++1e18-1e20 p++ 1e18-1e20

As can be seen in FIG. 7, the first doped slab region can be defined bya thickness d_(np1) by which it extends downwards into the slabs of thefirst material M₁. The first lower sidewall portion 713 b and secondlower sidewall portion 714 b each extend upwardly away from the slab bya height h₃ which corresponds to the height of the lower portion of theridge. These lower sidewall portions 713 b, 714 b extend into the ridgeby respective distances d_(np2), d_(pp2), each of these respectivedistances being less than half the total cross-sectional width of thelower ridge portion, such that an undoped region separates the n++region from the p++ region thereby forming a p-i-n junction.

An electrical contact (not shown) will be located at each of the slabregions in order to apply a bias across the junction which is formed bythe doped regions. These electrical contacts will be located directlyonto the slab (i.e. at the upper surface of the lab, on either side ofthe ridge). Typically the contacts may be equidistant from therespective sidewalls of the ridge.

The first and second upper sidewall portions 713 c, 714 c extend intothe upper ridge portion of the ridge by a distance d_(n), d_(p)respectively, each of which is less than the respective distanced_(np2), d_(pp2), by which the lower sidewall portions 713 b, 714 b eachextend into the lower portion 712 a of the rib waveguide. Examples oftypical measurements are given (in nm) in Table 2:

TABLE 2 Geometry Tolerance h₁ [nm] 100-800 h₂ [nm] 100-400 h₃ [nm] 0-400 d_(np1), d_(np2) [nm]  50-300 d_(pp1), d_(pp2) [nm]  50-300 d_(p)[nm]  50-300 d_(n) [nm]  50-300

In this example, the waveguide device takes the form of a waveguideelectro-absorption modulator (EAM). However, it is possible that thedevice could instead take the form of another optoelectronic componentsuch as a waveguide photodiode (PD).

The structure of the device and its method of manufacture are similar tothat disclosed in U.S. 62/429,701, the entire contents of which isincorporated herein by reference.

FIG. 8 shows an optically active region which is similar to that shownin FIG. 7. However, the region indicated within dotted line 801 whichincludes the first doped slab portion 713 a, the first lower sidewallportion 713 b, and the first upper sidewall portion 713 c is formed ofcrystalline or amorphous silicon. Whilst not shown, it is possible thatthere is a buried oxide layer beneath the base 701 but this is optional.

FIG. 9 is similar to FIG. 8, except that the region indicated withindotted line 901 now includes the second doped slab portion 714 a, thesecond lower sidewall portion 714 b, and the second upper sidewallportion 714 c and so these regions are also formed of crystalline oramorphous silicon. Whilst not shown, it is possible that there is aburied oxide layer beneath the base 701 but this is optional.

FIG. 10 shows a variant optically active region to those previously.Here, the entire slab, as well as a portion of the ridge 713 b 714 b, iswithin the region indicated within dotted line 1001. This region isformed of crystalline or amorphous silicon. This device is shown with anoptionally buried oxide layer 1002 below the region 1001. This is alsotrue of the devices shown in FIGS. 7, 8, 9, 11, and 12.

FIG. 11 shows an optically active region which is similar to that shownin FIG. 10. However here the dotted region 1101 extends up one side ofthe rib waveguide and so includes the first upper sidewall portion 713 cand so this is also formed of crystalline or amorphous silicon. FIG. 12is an extension of this, where the second upper sidewall portion 714 cis also included and so this is also formed of crystalline or amorphoussilicon.

FIG. 13 shows a further alternative example of the optoelectronicdevice. Here, as with previous embodiments, a slab 1301 and rib 1302form a ridge waveguide disposed above a silicon substrate 201. However,in contrast to previous examples, the cladding layer 203 is not as wideas the slab 1301. Instead, first and second portions of a buried oxidelayer 202 a and 202 b are disposed underneath the slab 1301.

FIG. 14 shows a further alternative example of the optoelectronicdevice. Generally, this example is similar to any previous examplediscussed (and so may have, where appropriate, any of the featuresdisclosed with reference thereto). A difference however, shown in FIG.14, is that this device is provided on a double silicon-on-insulatorwafer. Therefore the cladding layer 1401, which confines an optical modeof the device to the optically active region 219, is above a substrate1402 (which is generally formed of silicon). That substrate 1402 isabove a buried oxide layer 1403, for example SiO₂, which is in turnabove a second substrate 1404. The manufacturing steps described aboveapply equally here, where etching is performed at least to a buriedoxide layer no longer shown in FIG. 14 but which would have been abovesubstrate 1402.

As used herein, a “substrate-based” optoelectronic device on a substrateis an optoelectronic device having an active region that is joined by acontinuously crystalline path to the substrate, e.g., one that is formedby one or more (e.g. epitaxial) crystalline deposition methods the firstof which involves growing crystalline material on the substrate, usingthe substrate as a crystal seed. As such, a substrate-basedoptoelectronic device is distinct from an optoelectronic device havingan active region that is separated from the substrate by anon-crystalline insulating layer such as an amorphous buried oxide (BOX)layer.

As used herein, the word “or” is inclusive, so that, for example, “A orB” means any one of (i) A, (ii) B, and (iii) A and B. It will beunderstood that when an element or layer is referred to as being “on”,“connected to”, “coupled to”, or “adjacent to” another element or layer,it may be directly on, connected to, coupled to, or adjacent to theother element or layer, or one or more intervening elements or layersmay be present. In contrast, when an element or layer is referred to asbeing “directly on”, “directly connected to”, “directly coupled to”, or“immediately adjacent to” another element or layer, there are nointervening elements or layers present.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

All references referred to above are hereby incorporated by reference.

LIST OF FEATURES

-   100 Chip-   101 Input waveguide-   102,107 Light guiding direction-   103 Input waveguide/OAR interface-   104 Optoelectronic device-   105, 205 OAR-   106 Output waveguide-   108 OAR/Output waveguide interface-   201 Silicon substrate-   202 a, 202 b Buried oxide-   203, 416 Cladding layer-   206 Capping layer-   207 a, 207 b Silicon-on-insulator layer-   208 First doped region-   209 Second doped region-   210 First heavily doped region-   211 Second heavily doped region-   212 First mask-   213 Cavity-   214 Upper surface of substrate-   215 a, 215 b Insulating liner-   217 Grown optically active region-   218 Second mask-   219 Ridge of rib waveguide-   220 a, 220 b Slabs of rib waveguide-   221 Capping layer-   222 Third mask-   223 First dopant implantation-   224 Fourth mask-   225 Second dopant implantation-   226 Fifth mask-   227 Third dopant implantation-   228 Sixth mask-   229 Fourth dopant implantation-   230 Seventh mask-   231 a, 231 b Via opening-   232 a, 232 b Electrodes-   401 Seed layer

What is claimed is:
 1. A system, comprising: a silicon-on-insulatorchip, an optical detector, an optical modulator, and an electroniccircuit connected to the optical detector and to the optical modulator,wherein the silicon-on-insulator chip comprises: a substrate, aninsulating layer, on the substrate, and a silicon layer on theinsulating layer, wherein: the optical detector is an optoelectronicdevice having an optically active region that is joined to the substrateby a continuously crystalline path, or the optical modulator is anoptoelectronic device having an optically active region that is joinedto the substrate by a continuously crystalline path, wherein theoptoelectronic device comprises: a cladding layer, on the substrate; andthe optically active region, on the cladding layer, and wherein: thecladding layer is crystalline and has a refractive index which is lessthan a refractive index of the optically active region, the insulatinglayer is directly on the substrate, over a first region of thesubstrate, and the cladding layer is on the substrate, over a secondregion of the substrate, different from the first region of thesubstrate.
 2. The system of claim 1, wherein the optically active regionis formed of Si_(x)Ge_(1-x-y)Sn_(y) where 5%≤x≤20%, 1%≤y≤10%.
 3. Thesystem of claim 1, wherein the cladding layer is formed of silicon orSiGe and is epitaxially grown silicon or SiGe.
 4. The system of claim 1,further comprising a silicon seed layer between the cladding layer andthe optically active region.
 5. The system of claim 1, wherein theoptically active region comprises a silicon-germanium region.
 6. Thesystem of claim 1, wherein a length of the optically active region isbetween 30 μm and 60 μm.
 7. The system of claim 1, wherein: theinsulating layer is disposed on a first or second horizontal side of thecladding layer, and the cladding layer has a height from the substratesubstantially equal to or greater than that of the insulating layer. 8.The system of claim 1, wherein the optically active region is disposedwithin a cavity of the silicon layer on the insulating layer which isdisposed above the substrate.
 9. The system of claim 1, furthercomprising: an input waveguide, coupled to a first side of the opticallyactive region; and an output waveguide, coupled to a second side of theoptically active region, wherein the interface between the inputwaveguide and the optically active region and the interface between theoutput waveguide and the optically active region are at an angle greaterthan 0° relative to a guiding direction of the input waveguide or theoutput waveguide.
 10. The system of claim 1, wherein the opticallyactive region includes a waveguide ridge, and has: an upper surface anda lower surface; a lower doped region, located at or adjacent to atleast a portion of the lower surface of the optically active region, andextending laterally outwards from the waveguide ridge in a firstdirection; an upper doped region, located at or adjacent to at least aportion of the upper surface of the waveguide ridge of the opticallyactive region, and extending laterally outwards from the waveguide ridgein a second direction; and an intrinsic region located between the lowerdoped region and the upper doped region.
 11. The system of claim 1,wherein the optically active region includes a rib waveguide modulationregion, the rib waveguide modulation region having: a ridge extendingfrom the cladding layer; a first slab region at a first side of theridge and a second slab region at a second side of the ridge; andwherein: a first doped region extends along the first slab region andalong a first sidewall of the ridge, the first sidewall contacting thefirst slab region; and a second doped region extends along the secondslab region and along a second sidewall of the ridge, the secondsidewall contacting the second slab region.
 12. The system of claim 1,wherein the optically active region includes a rib waveguide modulationregion, the rib waveguide modulation region having: a ridge extendingfrom the cladding layer, at least a portion of the ridge being formedfrom a semiconductor material which is different from the material ofthe cladding layer; wherein the cladding layer includes a first slabregion at a first side of the ridge and a second slab region at a secondside of the ridge; and wherein: a first doped region extends along thefirst slab region and along a first sidewall of the ridge, the firstsidewall contacting the first slab region; and a second doped regionextends along the second slab region and along a second sidewall of theridge, the second sidewall contacting the second slab region.
 13. Thesystem of claim 1, wherein the optically active region includes a ribwaveguide modulation region, the rib waveguide modulation region having:a ridge extending from the cladding layer, at least a portion of theridge being formed from a semiconductor material which is different fromthe material of the cladding layer; a first slab region at a first sideof the ridge and a second slab region at a second side of the ridge; andeither the first slab region or the second slab region is the materialof the cladding layer; and wherein: a first doped region extends alongthe first slab region and along a first sidewall of the ridge, the firstsidewall contacting the first slab region; and a second doped regionextends along the second slab region and along a second sidewall of theridge, the second sidewall contacting the second slab region.
 14. Thesystem of claim 1, wherein the optically active region includes a ribwaveguide modulation region, the rib waveguide modulation region having:a silicon base, disposed on top of the cladding layer; a ridge extendingfrom the silicon base, at least a portion of the ridge being formed froma semiconductor material which is different from the material of thesilicon base; wherein the silicon base includes a first slab region at afirst side of the ridge, and a second slab region at a second side ofthe ridge; and wherein: a first doped region extends along the firstslab region and along a first sidewall of the ridge, the first sidewallcontacting the first slab region; and a second doped region extendsalong the second slab region and along a second sidewall of the ridge,the second sidewall contacting the second slab region.
 15. The system ofclaim 1, wherein the optically active region includes: a waveguideridge; and a waveguide slab, wherein at least one of: a sidewall of thewaveguide ridge; a portion of the waveguide slab; the entirety of thewaveguide slab; a portion of the waveguide ridge adjacent to thewaveguide slab; and both sidewalls of the waveguide ridge; are formed ofcrystalline or amorphous silicon and contain dopants.
 16. The system ofclaim 15, wherein the remainder of the waveguide ridge and waveguideslab is formed of SiGe, germanium, or SiGeSn.
 17. The system of claim 1,wherein the optically active region includes a rib waveguide modulationregion, the rib waveguide modulation region having: a ridge extendingfrom the cladding layer, at least a portion of the ridge being formedfrom the material of the cladding layer; wherein the cladding layerincludes a first slab region at a first side of the ridge and a secondslab region at a second side of the ridge; and wherein: a first dopedregion extends along: the first slab region and along a first sidewallof the ridge, the first sidewall contacting the first slab region; and asecond doped region extends along: the second slab region and along asecond sidewall of the ridge, the second sidewall contacting the secondslab region.
 18. The system of claim 1, further comprising: a firstarrayed waveguide grating (AWG) comprising a plurality of waveguides ina plane, one or more inputs, and one or more outputs, wherein: theoptical detector is configured to convert an input modulated opticalsignal to an electrical signal, the electronic circuit is configured todrive the optical modulator with a modulator drive signal based at leastin part on the electrical signal, the optical modulator is configured toreceive unmodulated light and to produce an output modulated opticalsignal having a modulation based on the modulator drive signal, theoptical modulator is located within the plane, the optical detector andthe optical modulator are located at an input or at an output of thefirst AWG, and an input waveguide for the input modulated optical signallies within the plane.
 19. The system of claim 18, wherein the opticaldetector is located within the plane.
 20. A system, comprising: asilicon-on-insulator chip, an optical detector, an optical modulator,and an electronic circuit connected to the optical detector and to theoptical modulator, wherein the silicon-on-insulator chip comprises: asubstrate, an insulating layer, on the substrate, and a silicon layer onthe insulating layer, wherein: the optical detector is a substrate-basedoptoelectronic device on the substrate, or the optical modulator is asubstrate-based optoelectronic device on the substrate, wherein theoptoelectronic device comprises: a cladding layer, on the substrate; andan optically active region, on the cladding layer, and wherein: thecladding layer is crystalline and has a refractive index which is lessthan a refractive index of the optically active region, the insulatinglayer is directly on the substrate, over a first region of thesubstrate, and the cladding layer is on the substrate, over a secondregion of the substrate, different from the first region of thesubstrate, the system further comprising: a first arrayed waveguidegrating (AWG) comprising a plurality of waveguides in a plane, aplurality of inputs, and a plurality of outputs, wherein: the opticaldetector is configured to convert an input modulated optical signal toan electrical signal, the electronic circuit is configured to drive theoptical modulator with a modulator drive signal based at least in parton the electrical signal, the optical modulator is configured to receiveunmodulated light and to produce an output modulated optical signalhaving a modulation based on the modulator drive signal, the opticalmodulator is located within the plane, the optical detector and theoptical modulator are located at an input or at an output of the firstAWG, and an input waveguide for the input modulated optical signal lieswithin the plane.